1. Field of the Invention
The present invention relates to control transmissions with hydraulic and mechanical gearing, especially transmissions which are used in commercial utility vehicles.
2. Description of the Related Art
All gear shifts should be smooth under load, regardless of the type of engine or differential transmission that is being applied. These requirements for a transmission control unit are best addressed with an electrohydraulic control system whose operating principle is described in the technical publication xe2x80x9cHydrodynamik in der Antriebstechnikxe2x80x9d, published by J. M. Voith GmbH, Vereinte Fachverlage Krauskopf-Ingenieur Digest, Mainz, 1987, in the German Patent Document No. DE 36 07 329, or in the paper by M. Bek, xe2x80x9cElektronisches Steuerungssystem fuer Automatgetriebexe2x80x9d Voith research and design, paper 33 (1989), Voith print G 1225 (3.89).
The control system, known from German Patent Document No. DE 36 07 329, is designed for multi-gear power shift transmissions for motor vehicles and includes pressure-actuated clutch elements in the form of either brakes or clutches which are used to engage and change the individual gears. The pressure supply to each of these clutch elements is controlled by a control valve. Furthermore, a control system is provided which determines the shift patterns of the transmission depending upon the various operating conditions, such as rotational input speed and/or rotational output speed of the transmission and the throttle position of the fuel system. The electrohydraulic control system known from the German Patent Document No. DE 36 07 329 is characterized by a control valve which is designed to combine the functions of an electromagnetic shift valve and a pressure regulator valve. The force exerted on the valve body, and causing the movement of same, by the electromagnet""s armature is adjustable by varying the strength of the magnetic field. The electromagnet is provided with a device for the formation of a measuring voltage which is proportional to the time-based change of induction, for the purpose of determining a measured variable (responsible for establishing the actual value) which, in turn, is needed for the instantaneous magnetic induction and the subsequent formation of the magnetic force. Connected to the device is at least one integral element which converts the measuring voltage into the measured variable that is proportional to the induction. Furthermore, a control element is provided to form a manipulated value for the exciting current for the electromagnet, by comparing the actual value derived from the integral element with the set point value provided by the control system. Thus, the magnetic force and the subsequent pressure on the hydraulic fluid acting upon the clutch elements is adjusted to the value corresponding to the set point value. Finally, a time element is provided which imposes a time-based limit on the control process at the initiation of a shift action of the transmission so that, subsequently, the actual value and/or the set point value assume a minimum or maximum value.
Electronic control system for such actuators, applied as solenoid valves for electro-hydraulic applications, are extensively described in M. Bek, xe2x80x9cElektronisches Steuerungssystemxe2x80x9d, Voith research and design, paper 33 (1989), Voith Druck G 1225 (3.89), the disclosure of which is fully incorporated by reference herein. The electrohydraulic control system from the above publication takes advantage of the relationship between the control pressure p of the hydraulic system and the force Fm of the electromagnet as described in the following equation:
p=Fm/Ak
whereby Ak is the effective area of the control piston. On the other hand, the force exerted by an electromagnet, assuming a small air gap, is approximated by this equation:
Fm≈1/2xcexc0A."PHgr"2
wherein:
xcexc0: Permeability of air
A: Area of the air gap
"PHgr": magnetic flux formed by the exciting current
In order to determine the actual value of the magnetic force Fm which is required for the electronic control of the magnetic force and, thus, the control pressure p, the magnetic flux must be determined. Concerning this subject matter, the publication (M. Bek, xe2x80x9cElektronisches Steuerungssystemxe2x80x9d, Voith research and design, paper 33 (1989), Voith print G 1225 (3.89), suggests introducing a measuring coil into the magnetic field of the field coil. According to the induction law, the (magnetic) flux can be obtained from the induced voltage through integration over time. The relationship between the magnetic force and the induced voltage is described as follows:             F      m        ⁡          (      t      )        ≈      (                  ∫        0        t            ⁢                                    U            m                    ⁡                      (            t            )                          ⁢                  xe2x80x83                ⁢                  ⅆ          t                    
Hence, in order to obtain the actual value of the magnetic force Fm the induced measured value absorbed by the measuring coil must be integrated.
Until now, the control of the magnetic force was performed by hardware-based analog controllers. The analog controllers were designed as discrete two-step controllers (ref. Dubbel, Reference book for mechanical engineering, Springer Verlag, Berlin, Heidelberg, New York, 1995, pages X8 through X9). The disadvantages of the analog controllers, as published in M. Bek, xe2x80x9cElektronisches Steuerungssystemxe2x80x9d . . . , whose control comparators, for example, were applied in form of connected operational amplifiers, are:
Batch-to-batch variability as a result of the integration behavior of the integrators which are used for the individual determination of Fm, making it necessary to balance the circuitry of the analog two-step controller by, for example, soldering resistors onto the appropriate integrator.
When applying a multiplex operation, which is the sequential control of several solenoid valves with one and the same electronic controller, the controller can only be balanced to one integrator.
Influences of the supply voltage +Ub on the magnetic force cannot be taken into account.
A further disadvantage of the analog controllers is the setting of the hysteresis, which is required for the stable operation of a two-step controller (ref. Dubbel, rest as stated above, page X8). The hysteresis is adjusted in these circuits by applying resistors into the circuit design, thereby making a subsequent change to the hysteresis value no longer feasible.
Further, the hardware circuitry assigns a discrete manipulated output value for every actual input value. This rigid relationship does not permit the control of two solenoid valves with one and the same controller.
With the present invention, the chosen gear befits the vehicle speed and engine load so that optimum engine operating conditions and driveability characteristics can be achieved all times, even under varying driving conditions.
The present invention provides an electronic control system for an electrohydraulic controller of a power shift transmission, as well as establishes a process for controlling such a transmission. The control system overcomes the above-stated disadvantages associated with the state of the art.
An electrohydraulic control system with electronic control for a transmission includes at least one actuator element, a device to accept a measured signal from one actuator element, or a device to accept a plurality of measured signals if one and the same electronic control system serves several actuator elements. The electronic control system further includes a device to form an actual value of the actuator element(s) from the received analog measured signal. This measured signal is used for the control of the actuator element(s). The electronic control system includes an A/D converter, which, in a first embodiment, is connected between the analog actual value generator and the processor, and serves to convert the analog actual value of the actuator elements into a digital actual value. This digital actual value is subsequently fed into a computing system, preferably a microprocessor. Alternatively, the A/D converter can be arranged to be ahead of a digital actual value generator. In such arrangements, the analog measured value is converted into a digital measured value. A digital actual value is then formed from the digital measured value in the digital actual value generator. The digital actual value is subsequently fed to a computing system. It is especially advantageous if the digital actual value generator, which, for example, can be an integrator, is integrated software-wise into the computing system such as a microprocessor.
With the help of a computer program stored in the computing system, a digital two-point control is performed by comparing the current converted digital actual value with the digital set point value which was stored in the processor by use of a cyclically commanded interrupt set point/actual value comparison. The manipulated signal obtained with this control logic is subsequently fed to an output driver for the control of the actuator element. In a special embodiment, provisions can be accommodated to again convert the manipulated signal from a digital into an analog signal by use of an digital-to-analog converter. A particular advantage of the digital signal processing in accordance to this invention, as compared to the currently available analog-based electronic control systems for electrohydraulic transmissions, is highlighted by the fact that tolerances associated with the components of the actual value generator or the amplifying circuits can be balanced quite easily and a scaling of the electronic control system can be performed. This is especially advantageous when several actuator elements need to be controlled by one and the same control system, as is the case, for example, in multiplex operation. In such a case, two-step control systems only have the capability to compensate and scale the tolerances of the components of only one channel, whereas the remaining channels have to remain unbalanced.
The scaling of the digital two-step controller can be accomplished quite simply with the help of a reference measurement. In a reference measurement, the value received for the electronic control is compared to a reference value and, based on this comparative assessment, a correction factor is established. This correction factor is preferably stored in a storage area of the processor. Since the correction factor takes into account the tolerances of the individual components, it remains, in this embodiment, constant for the electronic control system once its value has been established. A renewal of the correction factor is only necessary when, for example, repairs need to be performed on the control system, such as the replacement of the actual value generator.
It should be the preference to perform this correction on the set point value, since it changes at a substantially slower rate than the actual value. In doing so, it is possible to economize on computing capacity.
Since a preferred application of the invention-based electronic control of an electrohydraulic control system is intended for a transmission, the actuator elements are referred to as so-called solenoid valves, causing hydraulic pressure changes in the transmission, based on the signals from the electronic control system. In order to freely adjust the course of the hydraulic pressure, the solenoid valves are designed to include a pressure-regulating feature which includes an hydraulic control piston and an electromagnet connected to the top. By adjusting the electromagnetic force, it is possible to change the position of the hydraulic piston and, hence, adjust the pressure. Such pressure-regulating solenoid valves were published in M. Bek xe2x80x9cElektronisches Steuerungssystem fuer Automatgetriebexe2x80x9d, special publication by Voith research and design, paper 33 (1989), Voith print G 1225, page 5.6-5.7. The disclosure of this document is fully incorporated by reference herein.
In a first embodiment of this invention, the induced voltage signal of a measured coil is used as analog measuring signal for the magnetic force of the solenoid valve. The measuring coil is saturated by the magnetic field formed by the exciting current of the electromagnet""s field coil. Instead of a measuring coil, it is also possible to use a Hall-effect sensor or other devices known to the expert. In reference to the first embodiment, should the need arise to use the voltage signal induced by the measuring coil to infer the actual value of the magnetic force, it is necessary to integrate this analog voltage signal (ref. for example, M. Bek xe2x80x9cElektronisches Steuerungssystem for Automatgetriebexe2x80x9d, rest as stated above). In such an electronic control system, the analog measured signal will be fed into an actual value generator which is designed as an integrator. As a general rule, operational amplifiers are applied as integrators.
Depending upon the tolerances of the components used in the circuitry, the actual value generator obtains, correspondingly, a different magnetic force value at the same induced voltage signal. In order to achieve the same control characteristics with different components, it is necessary to account for the tolerances of these components, which is accomplished by performing reference measurements.
In addition to the apparatus, this invention also introduces a process for a digital two-step control of at least one actuator element of an electrohydraulic control system of a transmission, with the aid of an electronic control system. Reference measurements are performed for the purpose of determining correction factors for the electronic control of the actuator element(s). These reference value(s) are stored in a first storage area of the processor""s memory. The control process includes the following steps:
An analog measured signal from an actuator is received;
An actual value is formed from the analog signal for the actuator element;
The analog actual value is converted into a digital actual value by the A/D converter and is fed into a first storage area of the processor""s memory.
The digital two-step control, in a first version, occurs by comparing the digital actual input value in the processor to a corrected set point value, which is also stored in the processor. This comparative assessment then serves as a starting point for the subsequent steps. In another approach, the digital actual value is subtracted from the corrected digital set point value and, depending on the algebraic sign of the difference, the process proceeds to the next steps, accordingly. In a further solution, a digital hysteresis value, which is characteristic for this controller, is added to the corrected digital set point value. This forms the basis for the adjustment of the hysteresis for the two-step controller. In cases where the actuator element is applied in the form of a solenoid valve, it is possible, through suitable adjustment of the hysteresis on a software basis, to put the solenoid valve piston in a continuous state of micro-motion. This has the benefit of avoiding adverse static friction effects during the control process.
It is seen as an advantage for the sampling time to be in the area of 0.1 to 10 milliseconds. It is especially advantageous to work with a sampling period of one millisecond. A sampling period of approximately one millisecond makes it possible for the digital two-step controller to control the actuator sufficiently fast. At very low measured signal strengths, an amplifying circuit can be used to process this analog signal for the processor. If a magnetizing coil of a solenoid valve is used as an actuator element, it is considered part of the electrohydraulic control system for transmissions. These transmissions are, preferably, so-called torque divider transmissions.
In the section below, this invention is explained in more detail through the use of design examples and graphical representations: